Conventional techniques for cleaning the exhaust gas discharged from internal combustion engines and the like include the use of three-way catalysts as exhaust gas cleaning catalysts for simultaneously cleaning NOx, CO, and HC. When three-way catalysts are used to clean exhaust gas, the combustion reaction of the internal combustion engine must be controlled to the stoichiometric (theoretical) air/fuel ratio in order to simultaneously clean NOx, CO, and HC. Structures involving the use of oxygen-absorbing materials are also known in techniques employing such three-way catalysts. Such oxygen-absorbing materials absorb oxygen in environments having an excess of oxygen and release oxygen in environments where the oxygen concentration is depleted. The use of an oxygen-absorbing material in addition to a three-way catalyst in order to control the combustion to the stoichiometric level allows the catalyst environment to be kept in a substantially stoichiometric state, even when somewhat tardy control or the like results in slight deviations from the stoichiometric level, so as to ensure better catalyst function.
In another known technique, a mode for conversion to lean (excess air) combustion is provided in addition to the mode for stoichiometric operation of the internal combustion engine in order to improve the fuel consumption of the internal combustion engine. In this case, when lean combustion results in exhaust gas with excess air, three-way catalysts are generally unable to clean the NOx sufficiently. A structure that has thus been proposed (such as JP A 7-213902) is to add an NOx-absorbing material to the three-way catalyst so as to allow NOx produced during lean operations to be absorbed by the NOX-absorbing material. In cases where an NOx-absorbing material is thus provided in the exhaust gas cleaning device, when it is determined that the amount of NOx absorbed by the NOx-absorbing material is over a certain amount, the lean operation is temporarily suspended on behalf of a short period of fuel-rich operation (referred to as rich spike). As a result, the NOx absorbed to the NOx-absorbing material is released, and HC or CO produced by the combustion reaction during the rich spike function as reducing agents to clean the NOx. An oxygen-absorbing material may also be provide when using an NOx-absorbing material with a three-way catalyst to stabilize the exhaust gas cleaning performance in stoichiometric operating mode.
Ceria (CeO2) and the like have conventionally been used as oxygen-absorbing material in three-way catalysts. However, because conventionally known oxygen-absorbing materials such as CeO2 work at temperatures of up to about 400° C. when absorbing and releasing oxygen, oxygen cannot be sufficiently absorbed and released when the exhaust gas temperature increases such as during high loads (during high engine rpm, for example). In such cases, if there are delays or the like in control during stoichiometric combustion in the internal combustion engine, it is possible that the provision of an oxygen-absorbing material will not satisfactorily preserve the three-way catalyst performance.
In cases where an exhaust gas cleaning device comprising an NOx-absorbing element added to a three-way catalyst is used for lean operation, the temperature range within which NOx is absorbed and released by a conventionally know NOx-absorbing material such as an alkali metal is about 300 to 600° C., resulting in temperatures that overlap the temperatures at which the oxygen-absorbing material releases oxygen. As a result, rich spikes which are intended to clean NOx during lean operation may result in the release of NOx from the NOx-absorbing material and the release of oxygen from the oxygen-absorbing material. The reducing agents such as CO and HC which are produced during the rich spike react with the NOx released from the NOx-absorbing material as well as the oxygen released from the oxygen-absorbing material. Thus, to ensure that the NOx absorbed and released by the NOx-absorbing material is treated enough, it has been necessary to consume much of the fuel during rich spikes to obtain more HC and CO. As such, providing oxygen-absorbing materials may result in lower fuel consumption during rich spikes which occur during lean operation because of oxygen released from the oxygen-absorbing material.